"We think we know how stars like the Sun are formed, but there
are major problems in determining how a star 10 times more
massive than the Sun can accumulate that much mass. The new
observations with the VLA have provided important clues to
resolving that mystery," said Maria Teresa Beltran, of the
University of Barcelona in Spain.

Beltran and other astronomers from Italy and Hawaii studied
a young, massive star called G24 A1 about 25,000 light-years
from Earth. This object is about 20 times more massive than
the Sun. The scientists reported their findings in the
September 28 issue of the journal Nature.

Stars form when giant interstellar clouds of gas and dust
collapse gravitationally, compacting the material into what
becomes the star. While astronomers believe they understand
this process reasonably well for smaller stars, the theoretical
framework ran into a hitch with larger stars.

"When a star gets up to about eight times the mass of the Sun,
it pours out enough light and other radiation to stop the further
infall of material," Beltran explained. "We know there are
many stars bigger than that, so the question is, how do they
get that much mass?"

One idea is that infalling matter forms a disk whirling around
the star. With most of the radiation escaping without hitting
the disk, material can continue to fall into the star from the
disk. According to this model, some material will be flung
outward along the rotation axis of the disk into powerful
outflows.

"If this model is correct, there should be material falling
inward, rushing outward and rotating around the star all at
the same time," Beltran said. "In fact, that's exactly what
we saw in G24 A1. It's the first time all three types of
motion have been seen in a single young massive star," she
added.

The scientists traced motions in gas around the young star by
studying radio waves emitted by ammonia molecules at a frequency
near 23 GHz. The Doppler shift in the frequency of the radio
waves gave them the information on the motions of the gas. This
technique allowed them to detect gas falling inward toward a
large "doughnut," or torus, surrounding the disk presumed to
be orbiting the young star.

"Our detection of gas falling inward toward the star is
an important milestone," Beltran said. The infall of the
gas is consistent with the idea of material accreting onto
the star in a non-spherical manner, such as in a disk. This
supports that idea, which is one of several proposed ways
for massive stars to accumulate their great bulk. Others
include collisions of smaller stars.

"Our findings suggest that the disk model is a plausible
way to make stars up to 20 times the mass of the Sun. We'll
continue to study G24 A1 and other objects to improve our
understanding," Beltran said.

Beltran worked with Riccardo Cesaroni and Leonardo Testi of
the Astrophysical Observatory of Arcetri of INAF in Firenze,
Italy, Claudio Codella and Luca Olmi of the Institute of
Radioastronomy of INAF in Firenze, Italy, and Ray Furuya of
the Japanese Subaru Telescope in Hawaii.